1. Shyama Narendranath Space Astronomy Group ISRO Satellite Centre
Planetary exospheres
Shyama Narendranath
Space Astronomy Group
ISRO Satellite Centre
I the human wander lone in wonder
amid this grand universe of unbounded space and time
You the great keeper of universe
Exist in its infinite wonders
Lone in silence
In the grand home of your own being!
Rabindranath Tagore
2nd Venus Workshop October 2014

2. 2nd Venus Workshop 28-29 October 2014
Planetary exospheres
Exosphere: outermost part of a planet
Mean free path(l) of a molecule > pressure scale (H) height (l/H is called Knudsen number)
Collisions are rare, particles follow ballistic trajectories
Primarily light species (H, He, O)
Connects the planet to outer space
Lower boundary is called the exobase, upper boundary not defined
Atmospheric constituents escape from here
Hundreds of kms above the surface for Venus, Earth and Mars, starts from the surface for Moon and Mercury
Planet Rc
Vesc
Eesc (H)
Esc (O)
Earth
500
10.8
0.61
9.69
Venus
200
10.2
0.54
8.64
Mars
250
4.8
0.12
1.91
2nd Venus Workshop October 2014

3. 2nd Venus Workshop 28-29 October 2014
Escape Processes
Thermal (Jean’s escape)
Lighter gases such as H
Non-thermal
Collisions between exospheric constituents and energetic species (ions and electrons)
Dissociative re-combination, disscoiation by uv photons and electrons, charge exchange, plasma induced sputtering
Important mechanism of escape for heavier species such as O, N and C (thermal escape rate is small)
2nd Venus Workshop October 2014

4. 2nd Venus Workshop 28-29 October 2014
Modeling exospheres
Kinetic theory and individual particle dynamics
Hydrodynamic models
Collisionless atmosphere
Kn > > 1
Kn=1
Kn <<1
Escape flux ∞ Escape energy/thermal energy
Maxwellian distribution
Vertical distribution based on mass
Fluid like atmosphere
Turbulentmixing
Composition homogenous
2nd Venus Workshop October 2014

5. 2nd Venus Workshop 28-29 October 2014
Measurements
EUV spectroscopy
Emission lines from atmospheric constituents
H Lyman α at nm
H Lyman β at nm
He at 58.4 nm
O at nm
Insitu mass spectrometry
ENA imaging
X-ray spectrosocopy
Emission lines from excited solar wind ions as a result of collisions with exospheric neutrals in the 100 eV – 2 keV energy range
2nd Venus Workshop October 2014

6. 2nd Venus Workshop 28-29 October 2014
Venus exosphere
H corona of Venus: Mariner 5
Anderson, 1976
Day-night side asymmetry in temp- H bulge
Low exospheric temperatures ( K) even on the day side
Non-thermal escape processes may be dominant
2nd Venus Workshop October 2014

7. 2nd Venus Workshop 28-29 October 2014
D/H
Venus D/H = 150 x SMOW (upper atmosphere average)
Measurements
Lyman-α of H and D
NIR lines of H2O and HDO
Mesosphere- 250 times SMOW
HDO/H2O in Venus mesosphere from SOIR on Venus Express, Fedorova et al,2008
2nd Venus Workshop October 2014

8. X-ray spectroscopy of exosphere
Objective is to study the contribution of solar wind charge exchange to the atmospheric loss process
Charge exchange at Mars
Courtesy: Herbert Gunell, BISA
Work in progress
2nd Venus Workshop October 2014

9. 2nd Venus Workshop 28-29 October 2014
Summary
Atmospheric escape
Relative importance of thermal and the many non-thermal processes to be understood
Exospheric constituents, their temperature and density profiles required
Transition region at exobase and diffusion from troposphere to be understood
A suite- EUV spectrograph/Lyman α photometer, High resolution X-ray spectrometer, Mass spectrometer can measurements to understand loss processes in the exospheres
2nd Venus Workshop October 2014

10. 2nd Venus Workshop 28-29 October 2014